THE EFFECT OF THE IN VITRO COMBINATION OF CEFOXITIN AND CLAVULANIC ACID AGAINST GRAM-NEGATIVE BACILLI

One mechanism by which bacteria are resistant to antibiotics is the production of $\beta$-lactamase. Clavulanic acid, a $\beta$-lactamase inhibitor, has been used in combination with $\beta$-lactamase susceptible penicillins against Gram-positive and Gram-negative organisms. In this study, the effect of the combination of cefoxitin and clavulanic acid was assayed using the checkerboard titration method. Twenty clinical isolates were tested. Two criteria were used to evaluate the results: the fractional inhibitory concentration (FIC) index, and the reduction factor. An FIC index of $\leq$0.5 and a reduction in the MIC of both drugs of $\geq$4 times indicated synergy. No organism tested showed a synergistic reaction. Moreover, inconsistencies were found in the results when applying these criteria. No characterization of the $\beta$-lactamase was attempted. A reason for the absence of synergism in this study might be that these organisms produced Class I $\beta$-lactamase, which is not inhibited by clavulanic acid.